The present invention relates to high-pressure composite pipes to be used as high-pressure-resistant pipes or hoses which require a high internal pressure resistance strength.
The present invention also relates to a method for joining a pair of high-pressure composite pipes.
With regard to pipes for transporting a medium like water and gas, steel pipes are conventionally employed as such. While steel pipes exhibit a high internal pressure resistance strength and an excellent creep resistance, they are poorly quakeproof and develop rust and corrosion. In these days, therefore, synthetic resin pipes such as unplasticized polyvinyl chloride (PVC) pipes and polyethylene pipes are frequently employed.
A method for producing synthetic resin pipes is disclosed, for example, in Japanese Patent Application Laid-open No. H10-225988. A hollow article made of a crystalline thermoplastic resin is drawn by a clamp, from between a die and a former whose circumferential side surface is formed with a plurality of ridges having a section with a curvature radius of 0.5 mm or greater. Thereby, the pipe is formed as stretched in the axial and circumferential directions.
Synthetic resin pipes are exceptionally remarkable in terms of quakeproof property and impact resistance. On the other hand, they show a limited internal pressure resistance strength and a poor creep resistance. When a pipe element like a pipe or a hose is employed to transport various substances including liquid, gas, etc. in a flowing manner, the pipe element requires sufficient pressure resistance so as not to break under the pressure by an internal flow of substances. In particular, high internal pressure resistance is necessary for hydraulic oil piping, drain pipes and the like for carrying high-pressure fluids. From this point of view, Japanese Patent Application Laid-open No. H8-11250, for example, discloses a composite pipe which comprises a tubular inner layer and a tubular outer layer each made of a synthetic resin or other flexible materials, and which further includes a fiber reinforcing layer and a wire reinforcing layer interposed therebetween.
In this composite pipe, although the fiber reinforcing layer and the wire reinforcing layer improve the pressure resistance, they also cause following problems.
First of all, the resin reinforcing layer is prepared by braiding fibers of suitable thickness or by winding such fibers in spiral form. This process increases the thickness of the resin reinforcing layer by itself. Besides, in the resin reinforcing layer comprising fiber bundles, the strength in the circumferential direction which corresponds to the longitudinal direction of each fiber can be much greater than the strength in the axial direction. Further, because the wire reinforcing layer is laminated on the resin reinforcing layer, this structure increases the overall wall thickness and total weight, and thereby sacrifices the handlability and economical efficiency.
Secondly, when used to transport substances in a flowing manner, such composite pipes may fail to provide stable long-term service, depending on the physical properties and state of the substances to be transported. By way of example, when used as a hot-water pipe, a steampipe or like pipe for ships, or a chemical transport pipe or like pipe at chemical works, etc., an inner layer made of an ordinary synthetic resin may melt because of hot water, drug solution, etc., or deteriorate through chemical reactions. Furthermore, when used for transporting sand, ore and the like, an inner layer made of an ordinary synthetic resin may wear out and break only after a short service.
In addition, current demands include recycling of raw materials and associated economic effects. However, due to the complexity in separating the resin reinforcing layer and the wire reinforcing layer, recycling is a problematic task.
For the purpose of solving the above problems found in conventional technologies, the first object of the present invention is to provide high-pressure composite pipes which show an excellent pressure resistance and optimum applicability to various use, and also to provide lightweight high-pressure composite pipes which can be produced economically.
In another aspect, according to the conventional technologies for joining high-pressure composite pipes, they cannot be joined to each other directly, if the materials for the inner layer and the outer layer are different from those for the resin reinforcing layer and the wire reinforcing layer. Hence, such pipes have been joined by way of a pipe joint or the like. This technique has presented additional problems in terms of the strength, sealing property, etc. at the connection of the pipe joint and the high-pressure composite pipes. In summary, even if a high-pressure composite pipe has an excellent pressure resistance, any trouble in pipe arrangement can cause leakage or rupture at the connection of the high-pressure composite pipes.
For the pipe arrangement which includes a butt-fused connection area between the high-pressure composite pipes, attempts have been made to prevent damage at the butt-fused area under a load of internal pressure. For example, Japanese Patent Application Laid-open No. H11-101383 describes a method for bonding the exterior of the butt-fused area with a reactive resin. According to this method, after the butt fusion, a mold is mounted on the fused area, and a reactive resin is poured into the gap between the pipe and the mold. The resin is allowed to solidify for reinforcement.
However, as far as the reactive resin bonds the outer layers only, the outer layers can peel from the reinforcing layers. In this case, reinforcement is ineffective at the joint area of butt-fused high-pressure composite pipes. Besides, this method is troublesome from the viewpoint of construction, not only because the reactive resin should be handled with care but also because the mold should be removed after the resin has solidified. Further, since the solidified reactive resin is hard but brittle, it cracks easily when the high-pressure composite pipes are flattened.
With an intention of solving these problems, the second object of the present invention is to provide a method for joining high-pressure composite pipes, which method is capable of firmly joining high-pressure composite pipes having an excellent pressure resistance by a simple operation, firmly joining the high-pressure composite pipes by reinforcing the joint strength at the butt-fused area, and making a joint area adjustable to strain of the high-pressure composite pipes.
In order to achieve the first object mentioned above, the present invention includes the following arrangements.
A high-pressure composite pipe according to a first aspect of the present invention (hereinafter mentioned as a high-pressure composite pipe of Invention 1) comprises a pipe-shaped inner layer made of a synthetic resin, and a reinforcing layer which is made of a crosslinked stretched polyolefin resin sheet longitudinally stretched at a ratio of 10 or higher and which is wound on an external circumferential surface of the inner layer, with a winding direction of the reinforcing layer being oriented at a predetermined angle relative to an axis of the pipe.
The present invention utilizes a crosslinked stretched polyolefin resin sheet. With regard to the stretched polyolefin resin sheet used as a reinforcing sheet for a pipe, when the pipe is subjected to a circumferential stress generated by an internal pressure, the stretched polyolefin resin sheet, whose strength and elastic modulus are greater than those of the other layers (inner layer and outer layer) , needs to bear a greater share of the stress. For this reason, excellent creep property is an essential requirement. In order to enhance the creep property, a measure is taken to crosslink the stretched polyolefin resin sheet. A stretched polyolefin resin sheet is obtained by stretching a polyolefin resin, and shows a higher strength than a non-stretched polyolefin resin. However, this does not mean stretching improves the creep property. In fact, creep property is dependent on the molecular structure of a polyolefin resin to be used and not very much affected by stretching. Accordingly, when a stretched polyolefin resin sheet that has undergone stretching is crosslinked, the resulting stretched polyolefin resin sheet exhibits high strength and remarkable creep property.
The arrangement of Invention 1 provides a high-pressure composite pipe, in which the inner layer receives substantially uniform reinforcement by the reinforcing layer.
A high-pressure composite pipe according to a second aspect (hereinafter mentioned as a high-pressure composite pipe of Invention 2) comprises a pipe-shaped inner layer made of a synthetic resin, a reinforcing layer which is made of a crosslinked stretched polyolefin resin sheet longitudinally stretched at a ratio of 10 or higher and which is wound on an external circumferential surface of the inner layer, and an outer layer made of a synthetic resin and laminated on the reinforcing layer, with a winding direction of the reinforcing layer being oriented at a predetermined angle in a longitudinal direction of the pipe. The arrangement of Invention 2 provides an outer layer, with an intention of protecting the inner layer and the reinforcing layer mentioned in Invention 1 from an external force. The synthetic resin for the outer layer may be the same as, or different from, the one for the inner layer. Besides, the synthetic resins for the inner and outer layers may be, or may not be, fixed on the reinforcing layer by certain adhesion means. It is desirable, however, that the synthetic resins employed for the inner layer and the outer layer have adhesive properties to the reinforcing layer, in order to achieve a high internal pressure strength with a minimum strain of the high-pressure composite pipe when the high-pressure composite pipe is loaded with an internal pressure or external force.
The arrangement of Invention 2 provides a high-pressure composite pipe, in which the inner layer receives substantially uniform reinforcement by the reinforcing layer.
A high-pressure composite pipe according to a third aspect (hereinafter mentioned as a high-pressure composite pipe of Invention 3) comprises a pipe-shaped inner layer made of a synthetic resin, a reinforcing layer which is made of a crosslinked stretched polyolefin resin sheet longitudinally stretched at a ratio of 10 or higher and which is wound on an external circumferential surface of the inner layer, and an adhesion layer having affinity to the inner layer and the reinforcing layer and disposed between the inner layer and the reinforcing layer, with a winding direction of the reinforcing layer being oriented at a predetermined angle in a longitudinal direction of the pipe.
The arrangement of Invention 3 provides a high-pressure composite pipe, in which the inner layer receives substantially uniform reinforcement by the reinforcing layer. In addition, since the inner layer is fixed with the reinforcing layer by the adhesion layer, the reinforcing layer with a high elastic modulus suppresses strain of the inner layer when an internal pressure is imposed. Eventually, a high internal pressure strength is achieved.
A high-pressure composite pipe according to a fourth aspect (hereinafter mentioned as a high-pressure composite pipe of Invention 4) comprises a pipe-shaped inner layer made of a synthetic resin, a reinforcing layer which is made of a crosslinked stretched polyolefin resin sheet longitudinally stretched at a ratio of 10 or higher and which is wound on an external circumferential surface of the inner layer, an outer layer made of a synthetic resin and laminated on the reinforcing layer, an inner adhesion layer having affinity to the inner layer and the reinforcing layer and disposed between the inner layer and the reinforcing layer, and an outer adhesion layer having affinity to the outer layer and the reinforcing layer and disposed between the outer layer and the reinforcing layer, with a winding direction of the reinforcing layer being oriented at a predetermined angle in a longitudinal direction of the pipe.
The arrangement of Invention 4 provides a high-pressure composite pipe, in which the inner layer receives substantially uniform reinforcement by the reinforcing layer. In addition, since the inner layer and the outer layer are fixed with the reinforcing layer via the adhesion layers, the reinforcing layer with a high elastic modulus suppresses strain of the inner layer and the outer layer when an internal pressure is imposed. Thus, a high internal pressure strength can be achieved.
A high-pressure composite pipe according to a fifth aspect (hereinafter mentioned as a high-pressure composite pipe of Invention 5) comprises a pipe-shaped inner layer made of a synthetic resin, a first reinforcing layer which is made of a crosslinked stretched polyolefin resin sheet longitudinally stretched at a ratio of 10 or higher and which is wound in a circumferential direction, and a second reinforcing layer made of a crosslinked stretched polyolefin resin sheet and laminated along an axis of the pipe, with a winding direction of the first reinforcing layer being oriented at a predetermined angle relative to the axis of the pipe. The order of the first and second reinforcing layers is not critical. The arrangement of Invention 5 provides a high-pressure composite pipe, in which the inner layer receives a substantially uniform reinforcement by the first reinforcing layer. Besides, the second reinforcing layer remarkably enhances the rigidity in the axial direction of the high-pressure composite pipe.
A high-pressure composite pipe according to a sixth aspect (hereinafter mentioned as a high-pressure composite pipe of Invention 6) comprises a pipe-shaped inner layer made of a synthetic resin, a reinforcing layer which is made of a crosslinked stretched polyolefin resin sheet longitudinally stretched at a ratio of 10 or higher and which is wound on an external circumferential surface of the inner layer, an outer layer made of a synthetic resin and laminated on the reinforcing layer, and an insulating layer made of a synthetic resin foam and disposed between the inner layer and the reinforcing layer and/or between the outer layer and the reinforcing layer.
The arrangement of Invention 6 provides an insulating effect deriving from the insulating layer made of a foam. The resulting high-pressure composite pipe is suitable for a hot-water pipe and a water supply pipe.
Preferably, the polyolefin resin sheet is crosslinked to show a gel fraction of 20% by weight or higher. Also, it is desirable to crosslink the polyolefin resin sheet by adding a photopolymerization initiator and irradiating an electron ray or an ultraviolet ray.
In the high-pressure composite pipes of Invention 1 to Invention 6, the inner layer is preferably stretched in at least either of the axial direction or the circumferential direction of the pipe. Namely, the inner layer may be stretched only in either of the axial direction or the circumferential direction, or stretched in both of the axial and circumferential directions, which can be properly determined in accordance with the intended applications (hereinafter mentioned as a high-pressure composite pipe of Invention 7).
When the inner layer is stretched in the axial direction, molecules in the synthetic resin for the inner layer are oriented in the stretching direction, improving the strength and the modulus of elasticity in the stretching direction. As a result, the high-pressure composite pipe is relieved from axial deflection due to its own weight. When the inner layer is stretched in the circumferential direction, its tensile strength (i.e. strength against internal pipe pressure) is improved in the circumferential direction. The resulting high-pressure composite pipe manifests a high internal pressure resistance strength. Preferable stretching ratios for the inner layer are not lower than 1.1 in the axial direction and not lower than 1.3 in the circumferential direction. An axial stretching ratio of lower than 1.1 is ineffective, because the modulus of elasticity hardly improves in the axial direction. Also, a circumferential stretching ratio of lower than 1.3 fails to give any effect, in which case the improvement of the high internal pressure resistance strength is limited while the strength increases in the circumferential direction.
When the present invention refers to the stretching ratio of the inner layer, it should be understood that the stretching ratio in the circumferential direction means the ratio of outer diameters before and after stretching (OD after stretching/OD before stretching), and that the stretching ratio in the axial direction indicates the ratio of lengths before and after stretching.
Desirably, the inner layer, or each of the inner and outer layers, comprises a polyolefin resin (hereinafter mentioned as a high-pressure composite pipe of Invention 8). In this arrangement, the inner layer, or each of the inner and outer layers, shows affinity to the reinforcing layer made of a stretched polyolefin resin sheet, thereby ensuring high adhesion strength. Further, since the constitutive layers are made of a common material, the high-pressure composite pipe is readily recyclable.
In an arrangement comprising a pipe-shaped inner layer made of a synthetic resin, a reinforcing layer made of a stretched polyolefin resin sheet and wound on an external circumferential surface of the inner layer, and an outer layer made of a synthetic resin and laminated on the reinforcing layer, it is possible that at least one of the inner layer and the outer layer comprises a synthetic resin foam (hereinafter mentioned as a high-pressure composite pipe of Invention 9). According to this arrangement, the inner layer and/or the outer layer made of a synthetic resin foam acquire(s) an insulating effect.
In addition, at least one of the inner layer and the outer layer may include a plurality of hollow portions which extend along an axis of the pipe and which are spaced along a circumference of the pipe at a predetermined distance (hereinafter mentioned as a high-pressure composite pipe of Invention 10). When the inner layer includes the hollow portions, reinforcement is provided by partitions between neighbouring hollow portions. As a result, this arrangement ensures remarkable pressure resistance and also achieves weight reduction. Similarly, the hollow portions formed within the outer layer can accomplish weight reduction and economic efficiency.
Further, an external circumferential surface of the outer layer may be equipped with a plurality of ribs which project radially relative to an axis of the pipe and which are spaced along the axis at a predetermined distance (hereinafter mentioned as a high-pressure composite pipe of Invention 11). In this arrangement, the ribs reinforce the outer layer, by which the reinforcing layer is protected in a stable manner for a long period. In consequence, the reinforcing layer can effectively reinforce the inner layer without fail.
Preferably, in regard to the stretched polyolefin resin sheet for the reinforcing layer, the modulus of tensile elasticity in 0-2% strain range is lower than the modulus of tensile elasticity in 2-5% strain range (hereinafter mentioned as a high-pressure composite pipe of Invention 12). This arrangement is concerned with the pulsation pressure characteristics of the high-pressure composite pipe. In order to improve the rupture strength of a stretched polyolefin resin sheet, stretching should be conducted at a relatively high stretching ratio, which inevitably increases the modulus of elasticity. Above all, the modulus of initial tensile elasticity tends to increase significantly. Due to the high initial elastic modulus, when this stretched polyolefin resin sheet is wound on the inner layer to reinforce a high-pressure composite pipe, the stretched polyolefin resin sheet has to bear a heavy stress while the high-pressure composite pipe is subjected to a repetition of high internal pressures like pulsation pressure. Eventually, the stretched polyolefin resin sheet is likely to develop cracks, which induce rupture of the high-pressure composite pipe. In order to avoid this accident, the above arrangement specifies the treatment of the stretched polyolefin resin sheet such that its initial elastic modulus should be lower than the subsequent elastic modulus, whereby the stretched polyolefin resin sheet can obtain good stretchability. As a result, even when the high-pressure composite pipe is under a repetition of high internal pressures like pulsation pressure, the stretched polyolefin resin sheet receives a smaller stress, because it is readily stretchable at the initial stage. Thus, the stretched polyolefin resin sheet is improved in terms of flexibility to bending strain and durability against pulsation pressure.
The above-mentioned property is imparted to the stretched polyolefin resin sheet in the following manner, based on the principle described below.
The stretched polyolefin resin sheet is obtained in a highly stretched state. When this sheet is folded, fibrils in the sheet are easily dislocated and overlapped on each other. In view of this phenomenon, when the stretched polyolefin resin sheet is treated in the manner mentioned later, a multiplicity of fold structures can be created in the stretched polyolefin resin sheet. In this connection, the stretched polyolefin resin sheet goes through an extreme drop of the modulus of initial elasticity. While the elastic modulus rises later, the total stretchability increases in the end. The stretched polyolefin resin sheet is guided along a zigzag pass line composed of a combination of thin round bars having a diameter of about 10 mm. During this process, the above-mentioned phenomenon is successively repeated to develop multiple folds of fibrils within the stretched polyolefin resin sheet. The stretched polyolefin resin sheet obtained by this treatment shows a very low modulus of initial elasticity and a high rupture elongation. At the same time, the final rupture strength is unaffected and the rupture strength remains substantially unchanged, because this treatment has not broken molecular chains. To be more specific, the high-pressure composite pipe obtained by winding the stretched polyolefin resin sheet is fortified with a sheet with a low modulus of initial elasticity. Therefore, the resulting high-pressure composite pipe shows a remarkable flexibility to the initial load, improving its durability against pulsation pressure and the like. Besides, since the above treatment does not cause a significant change in the final rupture strength, sufficient performance is expected in terms of internal pressure resistance.
As for the structure of the reinforcing layer, the winding direction of the reinforcing layer may be inclined 30xc2x0 to 90xc2x0 relative to the axis of the pipe, and the reinforcing layer may be laminated on the inner layer and/or the outer layer at symmetric inclination angles relative to the axis of the pipe (hereinafter mentioned as a high-pressure composite pipe of Invention 13). More preferably, the winding angle (angle of inclination) of the reinforcing layer is in the range of 45xc2x0 to 70xc2x0. According to this arrangement, the high-pressure composite pipe shows a higher internal pressure strength, particularly against an internal pressure of a transported medium. In addition, a plurality of reinforcing layers may be braided (hereinafter mentioned as a high-pressure composite pipe of Invention 14). In this arrangement, the reinforcing layers are locked with each other to ensure a high internal pressure strength and rupture retardation.
Further, the reinforcing layer may be laminated on the inner layer and/or the outer layer, in such winding directions that winding angles relative to the axis of the pipe, in absolute value, are varied in the range of 5xc2x0 to 30xc2x0 (hereinafter mentioned as a high-pressure composite pipe of Invention 15). More specifically, when a plurality of stretched polyolefin resin sheets constitute the reinforcing layer, a winding angle is varied relative to an underlying longitudinal reinforcing layer. According to this structure, when a pressure is applied inside the high-pressure composite pipe, the stress transmitted to the stretched polyolefin resin sheets is dispersed in the directions of specified angles. As for the rupture internal pressure performance, the high-pressure composite pipe with winding angle variation is substantially as good as the one without winding angle variation. However, in actual use, the former retains its physical properties for a longer period, excelling in the pulsation pressure resistance property.
As for the adhesion layer, its material may be composed of an elastomer with a tensile elastic modulus in the range of 100 to 2000 kgf/cm2 (hereinafter mentioned as a high-pressure composite pipe of Invention 16). Use of the elastomer (i.e. soft resin) establishes a flexible joint with the reinforcing layer, so that the stress is not concentrated on a certain part of the reinforcing layer. Consequently, the high-pressure composite pipe can be improved in flexibility to bending strain as well as durability against pulsation pressure.
The soft resin used as the adhesion layer establishes a flexible joint with the reinforcing layer and improves the high-pressure composite pipe in terms of flexibility to bending strain and durability against pulsation pressure. Such effects are expected to hinder partial concentration of stress on the reinforcing layer.
As the elastomers, crosslinked rubbers are acceptable, but thermoplatic elastomers are preferable because of their advantage in heat adhesion. Such thermoplastic elastomers include styrene elastomers, vinyl chloride elastomers, polyester elastomers, urethane elastomers, polyolefin elastomers, etc. Above all, thermoplastic polyolefin elastomers are recommended, considering their high affinity particularly for polyolefin resins.
Among the physical properties of an elastomer, flexibility is measured by hardness and modulus of tensile elasticity. In this description, the modulus of tensile elasticity is taken as the measure. Where the tensile elastic modulus exceeds 2000 kgf/cm2, the elastomer shows a poor flexibility and possibly fails to achieve the above effects. On the other hand, below 100 kgf/cm2, the elastomer is too soft to function as the adhesion layer, only to cause dislocation of the reinforcing layer.
The material for the adhesion layer may have a crosslinked structure (hereinafter mentioned as a high-pressure composite pipe of Invention 17). The above-mentioned thermoplastic elastomers contain a so-called pseudo-crosslinked structure in which the crosslinked structure is lost by heating. Despite this pseudo-crosslinked structure, crosslinking is required if the heat resistance is deficient.
Polyolefin resins can be crosslinked, for example, by a chemical crosslinking method using a peroxide, a radioactive crosslinking method using electron rays or other high-energy radioactive rays, and a so-called silane crosslinking method effected by silane-graft treatment and moisuture contact. A crosslinking agent or a crosslinking auxiliary is blended in advance, especially when crosslinking is effected by irradiating an ultraviolet ray or an electron ray onto the stretched polyolefin resin sheet which has undergone stretching. Such crosslinking agents (radical photoinitiators) include benzophenone, thioxantone, acetophenone, benzyl-benzoine, Michler""s ketone. As crosslinking auxiliaries, there may be mentioned triallyl isocyanurate, triallyl cyanurate, trimethylolpropane acrylate, and diallyl phthalate. In the stretching step, these crosslinking agents also assist stretching by serving to reduce the pulling force (tensile force) required for the stretching.
In addition, the silane crosslinking method, which comprises simple steps, is also desirable.
In the present invention, the resin for the adhesion layer preferably shows a crosslinking degree in the range of 10 to 80%. On the one hand, heat resistance may not be enhanced under 10%. On the other hand, where crosslinking is continued after the crosslinking degree exceeds 80%, heat resistance does not improve any further.
The degree of crosslinking is evaluated in terms of gel fraction. In the present invention, the method for gel fraction measurement comprises the steps of heating a solution which is a good solvent to a polyolefin resin (i.e. principal constituent of the stretched polyolefin resin sheet), allowing insoluble matters to elute from the heated solution, and expressing the gel fraction as the weight percentage of the insoluble matters relative to the weight of the mixture before elution. For example, in the case of polyethylenes, elution is conducted for 24 hours in a xylene solution heated at 130xc2x0 C., and, thereafter, the gel fraction is determined as the weight percentage of the insoluble matters relative to the weight of the mixture before elution. In the case of polypropylenes, insoluble matters are obtained by 24 hour elution from a mesitylene solution heated at 135xc2x0 C., and the gel fraction is calculated as the weight percentage of the insoluble matters relative to the weight of the mixture before elution.
When the resin for the adhesion layer contains a crosslinked structure, the high-pressure composite pipe of the present invention can be employed at a high temperature over 80C., without sacrificing its performance.
 less than Stretched Polyolefin Resin Sheet greater than 
With regard to the above-mentioned high-pressure composite pipe of the present invention, description is made of the stretched polyolefin resin sheet which is used as a material for the reinforcing layer.
It should be understood that, in the present invention, the stretched polyolefin resin sheet refers to a sheet which is stretched at least in the axial direction of a pipe and which comprises a polyolefin resin as a principal material.
The polyolefin resin includes, but is not particularly limited to, for example, low-density polyethylenes, straight-chain low-density polyethylenes, high-density polyethylenes, homopolypropylenes, propylene random copolymers, propylene block copolymers, poly(4-methyl-1-pentene), etc. Among these polyolefin resins, polyethylenes (in particular, highly crystalline high-density polyethylenes) are advantageous for its high elastic modulus after stretching. Where necessary, polyolefin resins may be mixed with crystal nucleating agents, crosslinking agents, crosslinking auxiliaries, slipping agents, fillers, pigments, other kinds of polyolefin resins, low-molecular-weight polyolefin waxes, etc.
The crystal nucleating agent is added for the purpose of improving the degree of crystallization. For example, calcium carbonate, titanium oxide, etc. can be used as such.
The crosslinking agent and the crosslinking auxiliary are incorporated for partial crosslinking of molecular chains in the polyolefin resin, thereby improving heat resistance, creep property, etc. of the stretched polyolefin resin sheet. The crosslinking agent includes, for example, benzophenone, thioxanthone, acetophenone and other photopolymerization initiators. The crosslinking auxiliary includes triallyl cyanurate, trimethylolpropane triacrylate, diallyl phthalate and other polyfunctional monomers.
For a preferable arrangement for carrying out the crosslinking, a gel fraction is adjusted to not lower than 20%, which is the weight percentage of the residual matter after the non-eluted component is extracted from the stretched polyolefin resin sheet by using a good solvent to a polyolefin resin which principally constitutes the stretched polyolefin resin sheet. When the resin sheet comprises a high-density polyethylene as the main constituent, the solvent can be xylene, methylene, etc. In a resin sheet with a gel fraction of not lower than 20%, the resin sheet is chemically or physically crosslinked by 20% or more, and possesses sufficient creep characteristics to reinforce the inner layer for a long period. Since such characteristics protect the resin sheet from elution by solvents and melting under heating, the resin sheet does not lose its strength during the step of integrally laminating the resin sheet on the inner layer. Therefore, the resin sheet is allowed to exhibit a full reinforcing effect. There is no particular limitation to the process for giving the resin sheet with a gel fraction of not lower than 20%. By way of illustration, a photopolymerization initiator such as benzophenone, thioxanthone and acetophenone is blended as a raw material, and crosslinking is effected during or after the preparation of the stretched sheet.
In addition to the use of these crosslinking agents, electron ray or ultraviolet ray may be irradiated as a supplementary crosslinking method. The crosslinking method may include a process of irradiating an electron ray (preferably 1 to 20 Mrad, more preferably 3 to 10 Mrad) or irradiating an ultraviolet ray (preferably at an intensity of 0 to 800 mW/cm2, more preferably 100 to 500 mW/cm2) , after the polyolefin resin is blended with the crosslinking agent, the crosslinking auxiliaries, etc. as mentioned above. This crosslinking step can be conducted simultaneously with the stretching step to be described later, or after the stretching step.
After crosslinked by any of the above processes, the stretched polyolefin resin sheet is enhanced in creep property. Accordingly, while the high-pressure composite pipe is in service, improvement is observed in the creep property with respect to the internal pressure. In particular, if a polyolefin resin having a poor creep property is employed for the inner layer, it is desirable to crosslink the stretched polyolefin resin sheet.
The stretched polyolefin resin sheet is obtained by stretching a polyolefin resin sheet processed in sheet form. The method of preparing this polyolefin resin sheet is not particularly limited, and includes, for example, extrusion forming by T-die method, roll forming by calender method, etc.
In addition, there is no particular limitation to the method for continuous stretching of the polyolefin resin sheet. By way of illustration, a heated polyolefin resin sheet can be stretched between rolls rotating at different speeds. Alternatively, a heated polyolefin resin sheet is fed between rolls rotating in opposite directions and stretched in the axial direction of a pipe, with the thickness being gradually decreased (so-called rolling method). In these methods, the stretching step may be conducted only once, or repeated more than once for gradual stretching. If the stretching step is effected more than once, the above stretching methods may be performed in combination. Especially, in the case of a relatively thick polyolefin resin sheet, it is desirable to conduct the above-mentioned rolling prior to the stretching.
The thickness of the polyolefin resin sheet to be stretched (raw sheet material) is dependent on the use, stretching ratio, etc. of the intended high-pressure composite pipe. Although the thickness is not particularly limited, a desirable thickness is about 0.5 to 15 mm. With a thickness of less than 0.5 mm, the stretched polyolefin resin sheet is so thin as to He sacrifice its handlability and induce troubles in the subsequent operation steps (e.g. laminating operation) On the other hand, when the thickness exceeds 15 mm, excessive stretching load is imposed, only to enlarge the stretching machine needlessly and to complicate the stretching operation. From this raw sheet material, the stretched polyolefin resin sheet is obtained in a thickness of about 50 to 1000 xcexcm.
The width of the reinforcing layer composed of the stretched polyolefin resin sheet is not particularly limited. The width can be adequately selected according to the diameter, winding angle and winding process (mentioned below) of the high-pressure composite pipe. When the reinforcing layer needs to be in a relatively narrow width, a broad sheet can be slit into a required width.
The stretching ratio of the stretched polyolefin resin sheet is decided, as necessary, based on the properties and the state of the crystalline polyolefin resin used. Although this ratio need not be strictly specified, a preferred stretching ratio is not less than 10 in the longitudinal direction, and more preferably not less than 20. A stretched polyolefin resin sheet with a longitudinal stretching ratio of less than 10 may fail to provide a strength and an elastic modulus as required. Additionally, when stretching is effected in the width direction, stretching in the longitudinal direction is hampered. In this case, it may be difficult to carry out longitudinal stretching at a ratio of 10 or higher.
Where necessary, in order to enhance the adhesive property, the surface of the stretched polyolefin resin sheet may be treated by a physical or chemical method. Considering the simplicity of the operation, it is preferred to adopt a physical surface treatment of creating a microscopically uneven surface on the stretched polyolefin resin sheet. The surface treatment includes embossing methods like sand blasting and local heating methods of heating a part of the surface.
Further, the reinforcing effect of the stretched polyolefin resin sheet can be fully exhibited by attaching the stretched polyolefin resin sheet securely and tightly to the inner layer comprising a synthetic resin. If the stretched polyolefin resin sheet is not securely adhered to the inner layer, a gap created between the inner layer and the stretched polyolefin resin sheet impairs the reinforcing effect. Therefore, under a certain temperature condition for integrally fusion-laminating the stretched polyolefin resin sheet on the inner layer, the stretched polyolefin resin sheet is desired to show a longitudinal shrinkage stress of not less than 0.1 MPa. When the stretched polyolefin resin sheet has a shrinkage stress of less than 0.1 MPa, the integrating lamination process should be conducted under heating, while a tensile stress is applied on the stretched polyolefin resin sheet, which complicates the process. In contrast, when a stretched polyolefin resin sheet has a shrinkage stress of 1.0 MPa or greater, the inner layer and the stretched polyolefin resin sheet can be tightly fused together, simply by winding the stretched polyolefin resin sheet onto the inner layer and heating them at the above-mentioned temperature.
There is no strict limitation as to the manner of giving the stretched polyolefin resin sheet with a shrinkage stress of 0.1 MPa or greater. For example, an excessively stretched polyolefin resin film can be cooled rapidly, or crosslinking can be effected during or just after the stretching step.
Preferably, the melting point of the stretched polyolefin resin sheet is the same as or higher than that of the synthetic resin which is a main component of the inner layer. In the process of integrally laminating the stretched polyolefin resin sheet on the inner layer, the manufacture temperature should be not lower than the temperature at which the surface of the inner layer melts (i.e. melting point of a synthetic resin which principally constitutes the inner layer), in order that the inner layer and the stretched polyolefin resin sheet are fused tightly enough. On the other hand, if the stretched polyolefin resin sheet is melted, it loses the high strength derived from stretching and fails to exhibit a full reinforcing effect. Accordingly, as long as the stretched polyolefin resin sheet has the same or higher melting point than the synthetic resin which principally constitutes the inner layer, the stretched polyolefin resin sheet can be integrally laminated on the inner layer, without sacrificing its high strength.
There is no specific limitation to the manner for giving this melting point to the stretched polyolefin resin sheet. For example, provided the main constituent of the inner layer is a high-density polyethylene having a melting point of 135xc2x0 C., the stretched polyolefin resin sheet can be prepared by using a homopolypropylene with a melting point of 167xc2x0 C. as the principal constituent. Otherwise, while the main constituent of the inner layer is a high-density polyethylene having a melting point of 135xc2x0 C., a sheet which mainly comprises the same or different high-density polyethylene with a melting point of 135xc2x0 C. is stretched at a ratio of not less than 10, preferably not less than 20. In this way, even if the melting point of the stretched polyolefin resin sheet is equal to or lower than that of the synthetic resin used as the principal inner layer constituent, similar operations and effects can be expected in comparison with a sheet whose melting point is the same as or higher than the synthetic resin used as the principal inner layer constitutent. As stated above, it goes without saying that the stretched polyolefin resin sheet acquires a sufficient reinforcing effect, when processed at a stretching ratio of 10 or greater, preferably 20 or greater. Nevertheless, a highly stretched polyolefin resin sheet prepared at such a high stretching ratio is difficult to handle, because of its tendency to crack in the longitudinal direction. But where the thickness of the stretched polyolefin resin sheet is 100 xcexcm, cracking is restrained.
 less than Inner Layer greater than 
The next description relates to the inner layer in the high-pressure composite pipe of the present invention.
In the high-pressure composite pipe of the present invention, the inner layer serves to allow passage of a transported medium. Therefore, the species of the synthetic resin for the inner layer is suitably selected in accordance with the types of the transported medium. The examples include, but are not limited to, polyolefin resins similar to those employed for the stretched polyolefin resin sheet, polyvinyl chloride, polyamides, various rubbers, polyolefin elastomers, chlorinated vinyl chloride resins, fluororesins, crosslinked polyethylene resins, polyurethane resins, etc. Among them, chlorinated vinyl chloride resins show a prominent heat resistance and do not melt even when high-temperature fluid is transported. For this advantage, chlorinated vinyl chloride resins can provide stable and long service as a hot-water pipe for delivering hot water, a steampipe of a ship for delivering high-temperature steam, etc. From another aspect, owing to the excellent chemical resistance, fluororesins are suitable for a pipe arrangement for transporting drug solution and other chemical substances. In addition, crosslinked polyethylene resins are excellent in heat resistance and flexibility. Therefore, for a sheath pipe header-type piping system in a house, where a hot-water pipe or water supply pipe is led through the inside of a sheath pipe which is laid in a bent manner and connected to a header, a composite pipe having a crosslinked polyethylene resin inner layer can be utilized as the hot-water pipe or water supply pipe. Such a composite pipe can be inserted into the bent sheath pipe along its bent configuration. Further additionally, polyurethane resins, which excel in wear resistance, are suitable when the composite pipe is intended for transportation of solid substances like sand, gravel, coal, etc.
The wall thickness of the inner layer is properly decided in accordance with the type of transported medium, the internal pressure in use or the intended application.
Additionally, in consideration of recycling of high-pressure composite pipe fragments produced during manufacture or recycling of used high-pressure composite pipes, polyolefin resins are preferable as the synthetic resin for the inner layer.
 less than Outer Layer greater than 
The synthetic resin for the outer layer is optionally selected in accordance with the intended application, condition of use, etc. In addition to the above-mentioned synthetic resins for the inner layer, use can be made of polyamides, acrylic resins, polyester resins, etc. Similar to the inner layer, the wall thickness of the outer layer is properly determined in view of its intended application, condition of use, etc.
Additionally, in consideration of recycling of high-pressure composite pipe fragments produced during manufacture or recycling of used high-pressure composite pipes, polyolefin resins may be used as the synthetic resin for both the inner and outer layers.
 less than Adhesion Layer greater than 
When the stretched polyolefin resin sheet is stretched at a high ratio, crystals are so highly oriented on the surface that the surface cannot fuse readily with other materials in many cases. In order to secure the fixation of the inner layer on the reinforcing layer, or the fixation of the inner and outer layers on the interposed reinforcing layer, it is desirable to dispose a layer having affinity (compatibility) to each of these layers. The high-pressure composite pipe of Invention 3 includes an adhesive layer disposed between the inner layer and the reinforcing layer, and the high-pressure composite pipe of Invention 4 includes adhesive layers disposed between the inner/outer layers and the interposed reinforcing layer. Each of these adhesive layers has affinity (compatibility) to the related layers.
In this description, it should be understood that xe2x80x9cto have affinity (compatibility)xe2x80x9d is related to the adhesive performance of synthetic resins, indicating that the inner layer and the reinforcing layer, or the inner/outer layers and the interposed reinforcing layer, are substantially attached without peeling from each other, when they are pressed into contact and heated. Even though some synthetic resins may leave a small portion unadhered due to bubble inclusion or the like, as long as the high-pressure composite pipe is practically unaffected, they are encompassed in the synthetic resins having affinity (compatibility) to the reinforcing layer.
A preferable material for the adhesion layer is an elastomer with a tensile elastic modulus in the range of 100 to 2000 kgf/cm2, as mentioned above. Since reference to this elastomer is already made with regard to Invention 16, no further description is repeated here.
As the adhesion layer made of a synthetic resin, use can be made of a styrene-ethylene-butadiene-styrene copolymer (SEBS) , which is adhesive to the reinforcing layer and most of the above-mentioned synthetic resins. Where polyolefin resins are used for the synthetic resins for the inner and outer layers, there may be mentioned, for example, polyolefin resins used as the above-mentioned raw sheet material, copolymers comprising an olefin main chain (of which an olefin is the principal component) and other comonomers, acid-modified polyolefins, olefin elastomers, etc. These synthetic resins can be adhered, by thermofusion, to the reinforcing layer made of the stretched polyolefin resin sheet.
It is also possible to use adhesives such as epoxy-based adhesives and acrylic adhesives, curable resins and tackifying adhesives. Since this method enables adhesion without heating the stretched polyolefin resin sheet, the stretched polyolefin resin sheet is unlikely to lose its physical properties during manufacture. Further, use of adhesives or curable resins can prevent decrease of the strength in a relatively high temperature range between 50xc2x0 C. and 100xc2x0 C. However, this method is not very much expected to give a higher adhesive strength than thermofusion.
Now, additional description is made on the high-pressure composite pipe of Invention 5 which is mentioned previously.
In this high-pressure composite pipe, the inner layer is provided with a first reinforcing layer made of a stretched polyolefin resin sheet and wound in a circumferential direction, and a second reinforcing layer made of a stretched polyolefin resin sheet and laminated in an axial direction of the pipe. The term xe2x80x9ccircumferential directionxe2x80x9d as used herein means the winding direction which is oriented at a required angle relative to the axis of a pipe.
The second reinforcing layer in the high-pressure composite pipe of Invention 5 is intended to reinforce the pipe in the axial direction, thereby to reduce the deflection caused mainly by its own weight. For this purpose, the stretching direction of the stretched polyolefin resin sheet is preferably oriented along the axis of the pipe (i.e. at the inclination angle of 0xc2x0). In practice, however, the stretching direction may be slightly offset from the axis of the pipe, as far as the above object is achieved at such an inclination angle.
Preferably, the second reinforcing layer is laminated with no gap. For gapless lamination, the sheet width should be the same as the external circumference in the cross section of the high-pressure composite pipe. Nevertheless, it is acceptable if some gap is left after lamination. Additionally, a plurality of reinforcing layers may be used for lamination.
In the first reinforcing layer and the second reinforcing layer, the number of layers can be suitably decided, depending on the thickness and stretching ratio of the sheet, and the performances required of the high-pressure composite pipe. These reinforcing layers may be different in stretching ratio and thickness.
Whichever of the first reinforcing layer and the second reinforcing layer may locate on the inner side of the pipe. Instead, these reinforcing layers may be laminated alternately.
The high-pressure composite pipe of the present invention is obtained by winding a reinforcing layer in a laminating manner and as inclined at a predetermined angle relative to the axis of the pipe, thereby providing circumferential reinforcement and enhancing the strength against internal pressure. In order to increase the strength against internal pressure, the winding angle (angle of inclination) of the reinforcing layer is preferably in the range of 30xc2x0 to 90xc2x0, more preferably in the range of 45xc2x0 to 70xc2x0, relative to the axis of the high-pressure composite pipe.
Although the lamination angle of the reinforcing layer can be optionally selected, care should be taken when the reinforcing layer is laminated gaplessly, because the angle is dependent on the sheet width relative to the cross-sectional configuration of the high-pressure composite pipe. While gapless lamination is preferred, the reinforcing layer may be laminated in a slightly spaced manner or an overlapped manner.
The high-pressure composite pipe of the present invention may incorporate additional arrangements, unless the effects of the invention are not adversely affected.
For example, the high-pressure composite pipe of the present invention may include a synthetic resin layer disposed between two layers of the reinforcing layer which are laminated at symmetric angles.
Besides, each of the inner layer and the outer layer may comprise two or more laminated layers of different synthetic resins. Likewise, the reinforcing layer may be composed of two or more laminated layers.
When the reinforcing layer is laminated in two or more layers, the winding direction relative to the axis of the pipe may vary from layer to layer.
The sectional configuration of the high-pressure composite pipe is not particularly limited. A circular section and a near-square section with rounded corners are preferred, because of their good efficiency regarding internal pressure strength and external pressure strength relative to the weight. A complex sectional configuration bothers lamination of the reinforcing layer.
In the present invention, the diameter of the high-pressure composite pipe is not strictly restricted. Thus, the high-pressure composite pipe can be produced in a range from a relatively small-diameter pipe (10 mm to 30 mm in inner diameter), to a large-diameter pipe (300 mm to 500 mm). As for the inner layer, the axial cross-sectional configuration and the circumferential cross-sectional configration are not particularly specified.
The method for producing the high-pressure composite pipe of the present invention is not particularly limited. To give an example, a synthetic resin pipe for the inner layer is prepared in advance, and the reinforcing layer is laminated on the surface of the synthetic resin pipe.
The inner layer can be obtained in hollow shape by extrusion molding which is usually practised in the production of pipes and hoses.
Methods for circumferentially laminating the reinforcing layer include, but are not limited to, a so-called spiral winding process of winding the reinforcing layer at a desired angle, and a so-called braiding process of winding, in a braided manner, the reinforcing layers prepared in a relatively narrow width. A suitable method can be selected in accordance with various conditions such as the production amount, the production speed and the diameter of the high-pressure composite pipe.
The spiral winding process comprises continuously winding the reinforcing layer on the inner layer (as mandrel) at a fixed angle relative to the axis of the inner layer. In the course of winding, it is allowable that the reinforcing layer may be overlapped or slightly spaced, as far as the performance of the high-pressure composite pipe is not adversely affected. In the case where the reinforcing layer should be wound free from gap and overlap, the winding angle is decided in accordance with the width of the stretched polyolefin resin sheet and the outer diameter of the mandrel. Preferably, the reinforcing layer formed by the spiral winding process comprises an even number of layers, rather than an odd number of layers, such that each layer alternates at the same positive/negative angles relative to the axis of the high-pressure composite pipe.
The braiding process comprises winding, in a braided manner, a plurality of reinforcing layers of relatively small width. In terms of design, the resulting high-pressure composite pipe has an internal pressure strength substantially equivalent to the one obtained by the spiral winding-process. In addition, the following advantages can be noted.
First of all, the mandrel is unlikely to be dislocated during manufacture, because winding is effected under a tension evenly applied in the entire circumferential direction. This prevents creation of a gap between reinforcing layers and decreases the risk of weeping. Besides, the reinforcing layers are locked with each other to suppress local deflection and, as a result, improve the internal pressure strength. Moreover, if a transported medium imposes a high internal pressure on the high-pressure composite pipe, the reinforcing layer which is stronger than the inner layer moves in such a manner as to give a greater braiding angle, thereby delaying rupture of the high-pressure composite pipe. This effect is prominant when the inner layer is made of a synthetic resin with a relatively poor creep resistance.
It should be understood that the above remarks are based on the comparison with the spiral winding process, on the assumption that the reinforcing layer is wound at the same density. The winding process is adequately chosen in accordance with the forming speed and the diameter of the high-pressure composite pipe.
With regard to Invention 5, as a method for laminating the second reinforcing layer along the axis of the pipe, a second reinforcing layer feeder can be disposed along the axis of the pipe, such that the second reinforcing layer can be paid out on the inner layer or the first reinforcing layer. However, this method is not the only option, and a suitable method can be selected, considering various conditions such as the production amount, the production speed and the diameter of the high-pressure composite pipe.
Next, in order to achieve the above-mentioned second object, the method of the present invention for joining high-pressure composite pipes is arranged as described below.
A method for joining high-pressure composite pipes according to an eighteenth aspect of the present invention (hereinafter mentioned as a high-pressure composite pipe joining method of Invention 18) is a method for joining high-pressure composite pipes each comprising a pipe-shaped inner layer made of a synthetic resin, a reinforcing layer made of a stretched polyolefin resin sheet and wound on an external circumferential surface of the inner layer, and an outer layer made of a synthetic resin and laminated on the reinforcing layer. The method comprises the steps of: heating and melting an end of each high-pressure composite pipe; flaring the melted end of each high-pressure composite pipe, with a diameter thereof gradually enlarging toward an end face thereof; and butting the flared ends of both flared high-pressure composite pipes together and fusing internal circumferential surfaces of their inner layers, with their reinforcing layers being turned outwardly.
According to this arrangement, at a joint area of the mutually fused high-pressure composite pipes, the inner layers form a thick bead along the entire circumference, so that the high-pressure composite pipes are firmly joined together. Hence, in a pipe arrangement for a water supply pipe or the like in which high-pressure fluid flows, there is no fear of leakage or fracture at the joint area which is firmly joined along the entire circumference.
The above arrangement may further comprise the steps of: preparing an annular band which has an annular shape and one or more slits therein, in which band a groove runs circumferentially in the middle of an inner circumferential surface, and round-tipped ridges extend circumferentially along both side edges of the inner circumferential surface; approximately centering the annular band over a fusion point of the high-pressure composite pipes; mounting the annular band in such a manner that the groove can house a bulge which is formed by fusion and protrudes outwardly along the fusion point, and that tips of the ridges can contact, on both sides of the bulge, with external circumferential surfaces of the high-pressure composite pipes; and fastening the annular band.
Preferably, the material for the annular band should be flexible enough to adapt to strain. For example, there may be mentioned iron, stainless steel and other metals, polyolefin resins and other soft resins, and the like.
According to this arrangement, the annular band fixes the high-pressure composite pipes, such that its round-tipped ridges bite, in an indenting manner, not only into the outer layer but also into the whole pipe. As a consequence, when each pipe comprises a tubular inner layer made of synthetic resin, a reinforcing layer made of a stretched polyolefin resin sheet and wound around the external circumferential surface of the inner layer, and an outer layer made of synthetic resin and laminated on the reinforcing layer, the high-pressure composite pipes can be butt-fused together, with effectively reinforcing the joint area and making the joint area adaptable to strain of the high-pressure composite pipes. Accordingly, it is possible to prevent fracture at the joint area, in a pipe arrangement for a water supply pipe or the like in which high-pressure fluid flows.
In addition, it is possible to include the steps of approximately centering a heat-shrinkable reinforcing material over the fusion point of the high-pressure composite pipes, and shrinking the reinforcing material by heating.
In this arrangement, the high-pressure composite pipes are butt-fused together, when each pipe comprises a tubular inner layer made of synthetic resin, a reinforcing layer made of a stretched polyolefin resin sheet and wound around the external circumferential surface of the inner layer, and an outer layer made of synthetic resin and laminated on the reinforcing layer. In this state, the heat-shrunk reinforcing material can effectively reinforce the joint area by fixing its perimeter. At the same time, the joint area is made adaptable to strain of the high-pressure composite pipes. Accordingly, it is possible to prevent fracture at the joint area, when the pipes are laid as a water supply pipe or the like in which high-pressure fluid flows.
Furthermore, it is possible to include the steps of: removing a bulge formed along the fusion point of the high-pressure composite pipes; approximately centering a heat-shrinkable reinforcing material over an area from which the bulge is removed; and shrinking the reinforcing material by heating.
In this arrangement, the heat-shrunk reinforcing material is tightly fixed on the butt-fused joint area, so as to reinforce its perimeter more effectively.
The reinforcing material may be a tube-shaped element obtained by integrally winding a stretched polyolefin resin sheet into tubular form. This arrangement facilitates a reinforcing operation on the external circumference of the joint area where the high-pressure composite pipes are butt-fused to each other.
The tube-shaped element can be also obtained by spirally and integrally winding the stretched polyolefin resin sheet on an external circumferential surface of a core, in a plurality of layers which are oriented in reverse inclination directions from each other, and by removing the core thereafter. In this arrangement, the stretched polyolefin resin sheet for the tube-shaped element may be oriented either monoaxially or biaxially. The overall stretching ratio is preferably in the range of 10 to 40. The resins usable for the stretched polyolefin resin sheet are the same as those used for the stretched polyolefin resin sheet which constitutes the reinforcing layer of the high-pressure composite pipe.
In this arrangement, the strength of the reinforcing material prepared by thermally shrinking the tube-shaped element is increased not only in the circumferential direction but also in the axial direction. Accordingly, further effective reinforcement can be provided at the perimeter of the joint area where the high-pressure composite pipes are butt-fused to each other.
As the reinforcing material, use can be made of a tube-shaped element obtained by enlarging a diameter of a thermoplastic resin pipe, in which the resin at the joint area of the high-pressure composite pipes and a vicinity thereof is oriented in a circumferential direction.
According to this arrangement, the tube-shaped element is oriented in the axial direction by the extrusion forming and also oriented in the circumferential direction by enlargement of the diameter. Therefore, the heat-shrunk reinforcing material has its strength increased not only in the circumferential direction but also in the axial direction. Thereby, further effective reinforcement is provided at the perimeter of the joint area where the high-pressure composite pipes are butt-fused.
As the resin for the thermoplastic resin pipe which constitutes the tube-shaped element, polyethylene resins are suitable, for example. However, in addition to polyethylene resins, use can be made of other synthetic resins including polyolefin resins, polyvinyl chloride resins, polyamides and the like which are the same as the materials for the inner layer and the outer layer of the high-pressure composite pipe.
In the tube-shaped element, a suitable inner diameter is 1.05 to 1.2 times larger than the outer diameter of the high-pressure composite pipes to be joined. Preferably, the length of the tube-shaped element is at least 0.8 times longer than the outer diameter of the high-pressure composite pipes to be joined.
In this joining method, the means for heating the heat-shrinkable reinforcing material or tube-shaped element can be judicially selected from known heating means such as a hot air generator, a band heater, an infrared heater and the like. However, temperature control is required, because the orientation of the resin returns to the original state at an extremely high temperature. As a convenient temperature control method, a temperature-dependent discoloring tape is stuck to the reinforcing material, so that the heating condition of the reinforcing material can be managed by checking the degree of discolorization. By way of example, the orientation in the stretched polyethylene resin sheet returns to its original state, once the temperature exceeds its melting point, 135xc2x0 C. In this case, a discoloring tape whose color changes at 120xc2x0 C. can be utilized to make allowance.
When the reinforcing material or tube-shaped element is lacking in weatherability or susceptible to scratch and scar, it is advised to wrap an anti-corrosion tape made of a soft vinyl chloride resin, etc. around the external surface layer of the heat-shrunk reinforcing material or the tube-shaped element.